1. Field of the Invention
This invention relates to a pipe structure suitable for being laid from a vessel into the sea, to a pipeline including such a pipe structure and to methods of laying and using a pipeline including such a pipe structure.
2. Description of Related Art
As is well known, there is a requirement in the offshore oil and gas industry for a pipeline that can be laid on the seabed and is insulated so that the raw fluids extracted from a well can be transported through the pipeline without excessive change of temperature of those fluids. As a result there have been many proposals for pipe structures suitable for use in such pipelines. In such proposals a common structure comprises an inner pipe for conveying the fluid to be transported, insulating material around the inner pipe and an outer pipe around the insulating material. The insulating material provides thermal insulation of the inner pipe and is protected from loss of its insulating properties as a result of ingress of water by the outer pipe.
In some situations it is desirable, not to insulate the pipeline and inhibit any heat exchange of the surrounding sea with the fluid passing along the pipeline, but rather to promote heat exchange with the surrounding sea. Usually this is for the purpose of cooling the fluid in the pipeline although it may be for the purpose of warming the fluid (if the sea temperature is higher than the temperature of the fluid).
In one proposal for cooling the fluid passing along a pipeline a concrete trough, made up of a plurality of sections arranged in series, is placed on the seabed on a path along which a pipeline is to be laid, and a pipeline is then laid into the concrete trough. The concrete trough raises the pipeline off the seabed and is formed with cooling ducts through which seawater flows when the pipeline is in use, the flow of the seawater being promoted by the convection currents generated as a result of the temperature difference between the fluid conveyed along the pipeline and the surrounding seawater.
We have found that it is difficult to place the sections of the concrete trough in the correct position on the seabed and then to lay the pipeline correctly into the concrete trough. Also the concrete trough sections may shift relative to one another after the pipeline has been laid. Furthermore at the end of the life of the pipeline, the concrete trough sections have to be recovered separately if they are not to be left on the seabed.